The present invention relates to internal-combustion engines of the type comprising:
at least one induction valve and at least one exhaust valve for each cylinder, each valve being provided with respective elastic means that bring back the valve into the closed position to control communication between the respective induction and exhaust ducts and the combustion chamber;
a camshaft for operating the induction and exhaust valves of the cylinders of the engine by means of respective tappets, each induction valve and each exhaust valve being actuated by a cam of said camshaft;
in which at least one of said tappets controls the respective induction or exhaust valve against the action of said elastic return means via the interposition of hydraulic means including a hydraulic chamber containing fluid under pressure;
said hydraulic chamber containing fluid under pressure being connectable, via a solenoid valve, to an outlet channel for decoupling the valve from the respective tappet and causing fast closing of the valve under the action of respective elastic return means;
said hydraulic means further comprising a piston associated to the stem of the valve and slidably mounted in a guide bushing, said piston being set facing a variable-volume chamber defined by the piston inside the guide bushing, said variable-volume chamber being in communication with the hydraulic chamber containing fluid under pressure by means of an end aperture of said guide bushing, said piston having an end appendage designed to be inserted into said end aperture during the final stretch of the closing stroke of the valve in order to restrict the communication port between said variable-volume chamber and said hydraulic chamber containing fluid under pressure, so as to slow down the stroke of the valve in the proximity of its closing.
An engine of the type referred to above is, for example, described and illustrated in the European patent applications Nos. EP-A-0 803 642 and EP-A-1 091 097 filed by the present applicant.
The purpose of the present invention is to further improve the solutions previously proposed by the present applicant in order to render operation of the system for variable actuation of the engine valves as efficient and reliable as possible.
With a view to achieving this purpose, the subject of the invention is an internal-combustion engine having all the characteristics referred to above and further characterized in that the aforesaid piston for actuating the valve stem has its axis aligned with the axis of the respective tappet, and in that said tappet is slidably mounted in a tubular element which constitutes an integral prolongation of the guide bushing of the piston for actuating the valve stem.
Thanks to the above characteristic, the reliability of the system is improved with particular regard to the fact that the tubular element within which the tappet is slidably mounted may be stably secured in its mounting position (for example, by means of a fixing pin) in any operating condition of the engine, in contrast to what can occur in known solutions, where the tubular element within which the tappet is slidably mounted is a separate bushing screwed into a respective seat in the cylinder head of the engine, and is consequently subject to the risk of getting unscrewed.
According to a further characteristic, the tubular body, which is made of a single piece and defines both the guide bushing of the piston for actuating the stem and the tubular element for guiding the tappet, also defines within it the aforesaid pressure chamber and has at least one radial aperture for setting the said chamber in communication with a pipe for feeding oil under pressure. According to a further characteristic, reference means are provided for the correct angular position of the aforesaid tubular body in order to guarantee that the aforesaid radial aperture providing communication is aligned with the oil-feed pipe. This solution is more advantageous than the known solution, which did not envisage reference means for the angular mounting position, the tubular body being surrounded by a circumferential liner for communication with the oil feed so as to ensure setting-up of the communication for any angular mounting position.
As compared to the aforesaid known solution, the invention presents the advantage of enabling a substantial reduction of the space occupied by the oil under pressure, which makes it possible to bestow less elasticity on the system, with the consequent possibility of achieving higher engine r.p.m., this latter characteristic being particularly important, for example, in the case of an engine for a sports car.